Generic balances with a load changing device are disclosed in DE 296 07 731 U1 as part of the prior art that is discussed in said patent. This published document discloses comparator balances that have the primary task of determining in succession a weight value for a plurality of test weights and of comparing said weight value with a weight value of a reference weight that is determined accordingly. In this case it is not the determination of the absolute mass of the test weights that matters, but rather only the difference with respect to the absolute mass of the reference weight. The present invention also relates primarily to comparator balances, but can also be readily employed in common weighing devices, in which the determination of the absolute mass is of primary importance.
The German patent DE 40 23 483 A1 discloses a device for determining the dry substance content; and this device can accommodate a plurality of sample vessels with a turntable, which functions as a load changing device, and can place said sample vessels on the weighing pan of a balance in interaction with a lifting device.
The balance of the known device comprises a load cell, consisting of a mechanical weighing system and an appropriate electronic control unit, usually integrated into a housing, and a top-pan load receptor, which is coupled to a load introducing element of the weighing system. Above the balance there is a turntable, which is used in essence as a test weight magazine. The test weights are placed on said turntable, distributed over its periphery, at predetermined positions, so that by rotating the turntable said test weights can be moved into a position exactly above the load receptor of the balance. In addition to its rotary mobility, the turntable is height adjustable with respect to a common base of both the turntable and the balance. By lowering the turntable, the test weight, positioned above the load receptor, can be lowered onto the load receptor. In this case a finger-like carrier arrangement of the load receptor extends through the corresponding vertical openings in the turntable and, when the turntable is lowered even further, supports the test weight against the force of gravity. As a result, the load of the test weight rests on the load receptor and can be coupled into the weighing system with the load introducing element, so that the balance can determine a weight value for a test weight. The special design of the carrier arrangement of the load receptor and the vertical openings in the turntable is not disclosed in the cited document. However, the person skilled in the art knows about a number of configurations that can also be used in the context of the present invention. Thus, the carrier receptacle and the vertical openings may be formed, for example, in the shape of horizontally oriented forks with intermittent tines. It is also possible for the carrier arrangement to have vertically directed supporting fingers, which extend through the enclosed holes in the turntable.
The drawback with the known device is the enormous amount of effort that would have to be devoted to supporting the turntable. In order to be able to repeat the positioning of the test weights with a high degree of accuracy, it is necessary to minimize any and all backlash of the turntable, in particular, a tilt backlash. At the same time, however, the turntable has to be able to move not only rotationally, but also linearly in the vertical direction with minimal friction. In this case the task of achieving a backlash that is as near zero as possible while at the same time retaining low-friction mobility in two directions of motion involves extremely high requirements in terms of bearing engineering, an aspect that is reflected in the high cost of the device.
A precise and repeatable positioning of the test weights is necessary for a variety of reasons. From a metrological point of view an exact positioning is required to observe the required uncertainties when determining the mass. Off-center loading of the weighing system leads to so-called off-center load errors. Although these off-center load errors can be reduced through special additional modules for automatic centering of the test weights, even in this case a repeatable positioning with the load changer is a necessary prerequisite for mass comparisons having a low degree of measurement uncertainty.
However, an exact positioning of the test weights is also necessary for reasons relating to the handling of the weights. If the test weights are not transferred, for example, parallel to the weighing pan due to the poor alignment or backlash in the guides of the load changer, then the test weights may show signs of wear and lateral creep.